A properly operating immune system enables an organism to maintain a healthy status quo by distinguishing between antigens associated with the organism itself, which are allowed to persist, and antigens associated with disease, which are disposed of. Decades ago, Burnet proposed that the immune system's ability to distinguish between "self" and "non-self" antigens results from the elimination of self-reactive lymphocytes in the developing organism (Burnet, 1959, The Clonal Selection Theory of Acquired Immunity, Vanderbilt Univ. Press, Nashville, Tenn.). The phenomenon wherein an organism loses the ability to produce an immune response toward an antigen is referred to as "tolerance".
Over the years, a number of observations consistent with the clonal selection theory of tolerance have been documented. For example, genetically non-identical twin cattle, which share a placenta and are exposed to each other's blood cells in utero, fail to reject the allogeneic cells of their sibling as adults (Owen, 1945, Science 102:400). As another example, adult rodents that had been injected, at birth, with hemopoietic cells from a genetically distinct donor rodent strain were able to accept tissue transplants from that donor strain (Billingham et al., 1953, Nature 172:603; Billingham, 1956, Proc. R. Soc. London Ser. B. 239:44). However, in the early 1980's it was shown that the injection of minute amounts of antigen (namely an immunoglobulin expressing A48 regulatory idiotype) induced the expansion of helper T cells (Rubinstein et al., 1982, J. Exp. Med. 156:506-521).
The concept of tolerization is associated with the traditional belief that neonates are themselves incapable of mounting an effective immune response. It has been generally believed that neonates rely on maternal antibodies (passively transferred via the placenta) for immunity, until the neonate begins to synthesize its own IgG anti-bodies (at about 3-4 months after birth, in humans; Benjamini and Leskowitz, 1988, "Immunology, A Short Course", Alan R. Liss, Inc., New York, p. 65). In spite of the fact that passive immunity still plays a dominant role until 6-8 months after birth, the immune system gradually acquires the ability to mount adult-like immune responses.
More recently, several groups have reported findings that dispute the hypothesis that exposure to an antigen in early life disarms the ability of the immune system to react to that antigen.
Forsthuber et al. (1996, Science 271:1728-1730; "Forsthuber") suggest that the impaired lymph node response of so-called "tolerized" mice was an artifact caused by a technical inability to assess immune function. They reported that neonatal mice, injected with hen egg lysozyme (HEL) in Freund's incomplete adjuvant ("IFA") according to a protocol considered to induce tolerance in adults as well as neonates, displayed an impaired response in the lymph nodes consistent with tolerization. However, the spleen cells of these mice reportedly proliferated vigorously in response to HEL, a response previously unmeasurable due to technical limitations. The authors propose that neonatal injection did not tolerize, but rather induced functional memory cells that were detectable in spleen but not lymph nodes.
Sarzotti et al. (1996, Science 271:1726; "Sarzotti") report that inoculation of newborn mice with a high dose of Cas-Br-M murine leukemia virus ("Cas") does not result in immunological unresponsiveness, but rather leads to a nonproductive type 2 response which is likely to have a negative effect on the induction of mature effector cells. According to Sarzotti, clonal deletion of relevant CTL was not observed in mice infected at birth with a low dose of Cas.
Finally, Ridge et al. (1996, Science 271:1723-1726; "Ridge") proposes that previous reports of tolerance induction may have been associated with a relative paucity of antigen presenting cells. Ridge observed the induction of CTL reactivity in neonatal mice injected with antigen expressed on dendritic cells (which are so-called professional antigen presenting cells).
The use of nucleic acids as vaccines was known prior to the present invention (see, for example, International Application Publication No. WO 94/21797, by Merck & Co. and Vical, Inc., and International Application Publication No. WO 90/11092). It was not known, however, that such vaccines could be used to induce an immune response, including humoral and cellular components, in infant mammals.